The present invention relates to a deflection control type ink jet printing apparatus in which a jet of ink under supersonic vibration is ejected from a nozzle to separate into droplets at a predetermined position where a charging electrode is located to selectively charge the ink droplets and the charged ink droplets are deflected by a deflection electrode to impinge on a sheet of paper to reproduce data thereon. More particularly, the present invention is concerned with an ink jet printer of the type which deflects charged ink droplets to a desired deflection by suitably varying the pressure of ink.
It is known in an ink jet printer that deflections of charged ink droplets are affected by various factors such as the pressure and viscosity of ink, the charges on the ink droplets, the intensity of a deflecting electric field and the masses of the ink droplets. For instance, a change in ink temperature is directly reflected by that in ink viscosity and, therefore, in an amount of deflection. Immediately after a power supply to the printer, the ink temperature is low to maintain the flying velocity of ink droplets low. As the ink temperature progressively rises, the ink becomes less viscous to speed up the movement of ink droplets. The relation is hyperbolic in that the deflection decreases as the ink temperature increases. A decrease in deflection can be compensated for by either increasing a charge or decreasing an ink pressure. For such compensation, it has been customary to adjust an ink pressure or a charging voltage by detecting a deflection or flying velocity of ink droplets, such as disclosed in Japanese Patent Application No. 55-100918/1980 and U.S. Pat. No. 3,787,882.
However, various problems must be settled to control the deflection of charged ink droplets to an optimum value. For example, though a control may be made such that one ink droplet be deflected to an optimum deflection, a plurality of ink droplets fly one after another in practice so that a due countermeasure has to be taken against misdeflection originating from Coulomb's force air resistances and the like acting between adjacent ink droplets. In a modern ink jet printer, charge compensation coefficients or respective steps of reference charging voltage are predetermined to compensate for a distortion of a deflection path attributable to an electric field developed by the preceding charged ink droplet, or a disturbance to the deflection path due to Coulomb's force or irregular distribution of air resistances. Therefore, at least the charge compensation coefficients or the various steps of reference charging voltage should preferably be prevented from being affected by the deflection control. That is, such compensation should preferably be performed by calculation with constants or like processing regardless of the deflection control. Thus, a deflection control relying on a control of the ink pressure instead of the charging voltage will prevent a simultaneous shift of the charging voltage to enable the calculation with constants, and, thereby, facilitate a control of the charging operation for printing purpose.